This invention relates to magneto-optic data storage devices, more particularly, to writing new encoded data over old data in devices of that kind. Presently, most magneto-optic data storage devices require one rotation of the disk to erase old data before new data can be written. This arrangement is used because of the difficulty to switch the polarity of the magnetic field rapidly enough to correspond to the new data stream to facilitate directly overwriting old data.
In the prior art, U.S. Pat. No. 3,676,867 teaches an arrangement and interaction of a laser and magnetic-field-producing coil for magnetizing a thin film in an easy direction of magnetization normal to the substrate on which the thin film is deposited. The magnetic field, being weaker than the coercive force of the film, is used for erasing and creating reversed domains in the film at points which are simultaneously heated by a laser beam.
In addition, the prior art includes U.S. Pat. Nos. 4,340,914, 4,495,530 and 3,778,791 in which the magnetic field is controlled or modulated by a signal representing data. U.S. Pat. No. 4,495,530 actually teaches away from using an electromagnet for producing a bipolar magnetic field because of perceived high frequency limitations of such an arrangement. Instead, two other systems configurations for processing information recorded on a magneto-optic medium are described. In one, a pulsed laser is used with a steady-state DC magnetic field. In the other, the magnetic field is driven by the data signal which interacts with a continuous wave laser. Finally, in U.S. Pat. No. 4,472,748 describes a system in which a constant erasing light beam and a modulated recording light beam are focused on adjacent tracks and a reversible external magnetic field is applied to the recording medium. The polarity of the erased tracks alternates every other track. The magnetic field is reversed by switching the direct current supplied to the magnetic-field-generating coil at each revolution of the disk.
Substantial prior art exists which relates to forming circular domains with a laser beam in magnetically coercive materials. See, for example U.S. Pat. Nos. 3,786,452, 3,836,895, 3,787,825 and 3,836,897. However, use of patterns of such domains, including overlapping of successive circular domains to form variable-length elongated domains, for recording encoded data is not shown.